Pressure control unit



Feb. 3', 1970 r B. L. HOFFMAN 3,

I PRESSURE CONTROL UNIT Filed Sept. 28, 1965 2 Sheets-Sheet 1 INVENTOR.

Y fifimo L flaw-n0 Feb. 3, 1970 B. L. HOFFMAN 3,493,006

PRESSURE CONTROL UNIT FiledSept. 28, 1965 2 Sheets-$heet 2 INVENTOR. -wmeo 4. Abram BY I United States Patent ABSTRACT OF THE DISCLOSURE Hydraulic pressure system which supplies hydraulic fluid in one direction at a first, lower pressure and which absorbs hydraulic fluid in the opposite direction at a second, higher pressure irrespective of flow variations.

This invention relates to hydraulic systems and more particularly it concerns a novel hydraulic pressure unit suitable for use with multiple die punch presses.

Modern presses which draw or otherwise form various intricate shapes from sheet metal utilize complex multiple dies which are hydraulically actuated. These dies are made up of a plurality of die elements, each of which is connected to actuating pistons of various size. The actuating pistons in turn are supplied with hydraulic fluid at a common pressure so that when the die heads of the press are opened, or displaced from one another, the hydraulic pressure maintains the pistons and their corresponding die elements in fully extended position. As the die heads close upon each other with the sheet metal workpiece therebetween, the die elements become retracted in a certain predetermined sequence, depending, at least in part, upon the areas of the pistons actuating them. As the die heads are thereafter opened, the die elements begin to extend again in reverse sequence. 'It is this specially controlled sequence of retraction and extension of the various die elements which permits various intricate shapes to be produced in a single operation of the press.

The above described die arrangement presents certain requirements of the hydraulic system which supplies fluid under pressure to the actuating pistons. This system, for example, must be capable of supplying a sufficient quantity of hydraulic fluid and at a sufiicient pressure to cause full extension of all actuating pistons and their associated die elements when the die heads are opened. This pressure must not be too high however, otherwise when the press is opened, the die elements will move toward their extended position with such suddenness that the formed workpiece will be torn violently out of the dies and destroyed or severely damaged. On the other hand, the hydraulic system must be capable of absorbing all of the displaced fluid during the closing of the die heads; and furthermore, it must closely maintain the pressure of the fluid in the system at a very high value during this time so that the workpiece can be properly formed by the extended die elements.

In addition to the above, a further problem arises from the fact that the rapid operation of the press produces very large changes in volume rates of flow of the hydraulic fluid in the system. Such rapid changes in flow rate produce very serious hydrodynamic effects which adversely affect valve operation, and of course, seriously affect die operation. These adverse hydrodynamic effects include fluid foaming and cavitation, pressure reflections and surges, slow valve response time and unstable valve operation.

According to the present invention, there is provided a novel hydraulic system suitable for use in connection with multiple die punch presses of the type described above. This novel system supplies hydraulic fluid at a first lower pressure for extending the pistons in a multiple die press during the opening thereof, and it absorbs at a controlled higher pressure, the fluid displaced by the retraction of the hydraulic pistons during the closing of the die. The system moreover is capable of accommodating very sudden and very large changes in volumetric fluid flow rate, without corresponding changes in system pressure, and without the adverse hydrodynamic effects usually produced under such conditions.

According to the present invention there is provided a novel pressure unit comprising a body formed with a reservoir and a reservoir chamber communicating with the reservoir, an inlet-outlet chamber and a novel valving arrangement interconnecting the two chambers. A hydraulic line connects the inlet-outlet chamber with a die assembly or other utilization means.

Two passageways are provided between the chambers, the one being provided with a check valve allowing flow into the inlet-outlet chamber and the other being provided with a spool type control valve. The control valve is configured and arranged to be non-responsive to surges in the inlet-outlet chamber. A pressure passageway however is provided between the inlet-outlet chamber so that increases in actual pressure are communicated to the end of the spool type valve for actuating it.

The control valve itself is specially configured to dissipate surges; and it is provided with specially shaped cut-outs for controlling the rate of change of valve opening according to the displacement of the valve and for directing fluid flow in a balanced manner without foaming.

There has thus been outlined rather broadly the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form the subject of the claims appended thereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based may readily be utilized as a basis for the designing of other structures for carrying out the several purposes of the invention. It is important, therefore, that the claims be regarded as including such equivalent constructions as do not depart from the spirit and scope of the invention.

A specific embodiment of the invention has been chosen for purposes of illustration and description, and is shown in the accompanying drawings, forming a part of the specification wherein:

FIG. 1 is a side elevational view, partially cut away, showing the external configuration of apressure unit embodying the present invention;

FIG. 2 is a section view taken along line 2-2 of FIG. 1 and further showing a portion of a die arrangement hydraulically connected to the pressure unit of FIG. 1;

FIG. 3 is a view similar to FIG. 2, partially cut away, and showing the system in a different portion of its operation;

FIG. 4 is a fragmentary section view, taken along line 44 of FIG. 1;

FIG. 5 is a perspective view of a valve spool used in the embodiment of FIG. 1; and

FIG. 6 is a perspective view of a modified version of a valve spool, similar to that of FIG. 5.

FIG. 1 shows a pressure unit 10 which embodies the present invention. This pressure unit is made up of a body 12, on the top of which is mounted a cylindrical reservoir 14. A pressure gauge 16 is connected through a valve 18 3 the body for reading the pressure of the hydraulic uid which passes into and out from the unit.

The internal construction of the pressure unit 10 and s integration with utilization means is best seen in the :ction views of FIGS. 2 and 3. As shown in these figures, 1e cylindrical reservoir 14 rests upon the top of the body 2 with a reservoir gasket interposed to prevent leakge of fluid. A reservoir cap 22 covers the top of the resrvoir; and the entire assembly is held to the body 12 y means of an axially extending hold down rod 24 which ithreadedly engaged at one end to the body 12 and which asses through the cap 22 at its other end. A hold down ut 26 is threaded on the other end of the rod 24.

A glass sight gauge 28 is mounted along the outside of 1e reservoir 14 to provide a visual indication of the liquid :vel therein. Hydraulic fluid is supplied to the reservoir ia a fluid intake nozzle 30 threaded into the cap 22. For ormal operation, the reservoir is maintained at about wo thirds capacity so that the liquid level is in the vicinity f a phantom line 32 shown in the drawing. The fluid in he reservoir is maintained at a positive pressure (about 00-125 p.s.i.) during normal operation; and for this purose an air nozzle 34, connected to an air pressure supply not shown), is also fitted into the reservoir cap 22.

The body 12 is formed with a reservoir chamber 36 nd an inlet-outlet chamber 38. These chambers are aranged in side by side relationship and are separated by n inner wall 40. The reservoir chamber 36-opens into he reservoir 14, while the inlet-outlet chamber 38 opens nto a hydraulic line 42, connected to utilization means 0 be described more fully hereinafter. Upper and lower :onnecting passageways 44 and 46 extend through the nner wall between the chambers 36 and 38. A check 'alve 48 is provided to control the opening of the upper iassageway 44, while a control valve 50 is provided to :ontrol the opening of the lower passageway 46.

The check valve 48 is arranged to permit fluid flow rom the reservoir chamber 36 into the inlet-outlet chamer 38, but not in the reverse direction. The valve comarises a plug 52 threaded into a first outer wall 54 of he body 12. This plug supports a valve element '56 for eciprocal movement against and away from a valve seat nsert 58 press fitted into the upper connecting passagevay 44. A check valve bias spring 60 is arranged between he plug 52 and the valve element 56 to urge the element 0 a passageway closing position against the insert 58. [he check valve bias spring 60 however, is quite light, 1nd whenever the pressure in the inlet-outlet chamber 38 lrops below the reservoir pressure, the unbalance will )vercome the spring force and insert the check valve elenent 56 to allow fluid to flow out from the reservoir 14 :hrough the upper connecting passageway 44 and the inlet- Jutlet chamber to the hydraulic line 42 as indicated by the arrow A. An excess of pressure in the inlet-outlet cham- Jer 38, on the other hand, will only serve to seat the :heck valve element 56 more tightly so that no fluid can :ass back into the reservoir 14, through the upper conuecting passageway.

The control valve 50 comprises a control valve spool 62, shown in perspective in FIG. 5. This spool is cylin- :lrical in shape and includes two larger diameter end portions 64 and 66, and a smaller diameter intermediate portion 68. The end portions 64 and 66 are hollowed out, as indicated in FIGS. 2 and 3, to provide a pressure responsive chamber 70 and a spring seating chamber 72. These two chambers are maintained in fiuid communication by means of a central passageway 74 extending axially of the spool 6-2. The fluid flow velocity through this passageway is restricted to a predetermined valve by means of a metering orifice insert 76 press fitted into the passageway tion, acts in conjunction with the valve arrangement to subdue the effects of pressure transients or knocks so as to ensure complete and smooth control over valve operations at all times.

The valve spool 62 is also provided with a plurality of cut-outs 78 distributed circumferentially about the right hand larger diameter end portion 66 where it meets the smaller diameter intermediate portion 68. These cut-outs are of semi-cylindrical configuration with their axes extending radially outward from the axis of the spool 62. As will be seen in the description of operation of the device, these cut-outs serve to give smoother and more balanced control of fluid flow without surging or foaming at the high pressures, large volumes and rapid transients which must be handled by the system.

The first larger diameter end portion 64 of the valve spool 62 extends into an opening 80 in the first outer wall 54 of the body 12. Also the second larger diameter end portion of the spool extends through both the lower connecting passageway 46 in the inner wall 40, and through a further opening 82 in a second outer wall 84 in the body. The two openings 80 and 82 in the outer walls 54 and 84 of the body are aligned with and of the same diameter as the lower connecting passageway 46 through the inner wall 40.

The control valve spool 62 is thus reciprocally movable between a passageway closing position as shown in FIG. 2, where the second larger diameter portion fully covers the lower connecting passageway 46, and a passageway opening position as shown in FIG. 3 where oil can flow along the path indicated by the arrow B, from the inletoutlet chamber 38 over the smaller diameter intermediate portion 68 of the spool 62 and out through the cut-outs 78 into the reservoir chamber 36. A valve spool bias spring 86 fits into the spring seating chamber 72 of the valve spool 62, and urges the valve spool toward the left as shown in FIG. 2 to its valve closing position.

A pressure end cap 88 and an adjustment end cap 90 are bolted to the two outer Walls 54 and 84 respectively of the body 12, and they serve to cover the valve spool openings 80 and 82 in these walls. The pressure end cap 88, as shown in the fragmentary section view of FIG. 4, is provided with an internal pressure passageway 92 which, as shown, communicates between a location up inside the inlet-outlet chamber 38 and the pressure responsive chamber 70 of the control valve spool 62. Pressure changes in the inlet-outlet chamber 38 are communicated via the passageway 92 and act upon the left end of the spool in opposition to the force of the spool bias spring 86 to move the spool to the right as in FIG. 3 and open the lower connecting passageway 46 between the inlet-outlet chamber 38 and the reservoir chamber 36.

The adjustment end cap 90, as shown is provided with an adjustment valve 94 which communicates via an opening 96 with the spring seating chamber 72 of the valve spool 62, and through the metering orifice insert 76 and the central passageway 74 of the spool, to its pressure responsive chamber 70. The adjustment valve '94 includes a tubular valve seat 98 into which extends a conically shaped adjustable valve element 100. The adjustable valve element is biased against the seat 98 by an adjustment valve spring 102, the stress on which is adjustably controlled by an adjustment screw 104 threaded into the adjustment end cap 90. A lock nut 106 is provided on the adjustment screw for securing it at any given setting. As shown in the fragmentary section view of FIG. 4, the region beyond the adjustment valve 94 is in communication, via a feedback passageway 108, with the reservoir chamber 36.

There is also provided a safety valve 110 communicating between the inlet-outletchamber 38 and the reservoir 14. This safety valve comprises an opening 112 extending from the inlet-outlet chamber to the reservoir and seat 116 in which is seated a valve ball 118. The ball is pressed into the seat by means of a safety valve spring operating between the upper end of the housing 114 and a ball aligning element 120. The stress on the spring can be adjusted by an adjusting screw 122. In the normal case, this adjustment would be set so the valve would open and pass fluid into the reservoir only when the pressure in the inlet-outlet chamber rose above 5000 p.s.i.

The system is additionally provided with cooling means including a cooling coil 124 which extends in helical manner up through the inside of the reservoir. This cooling coil is also connected to various internal passageways 126 within the body 12 for passing coolant therethrough.

The inlet-outlet chamber 38 is connected via the hydraulic line 42 to a utilization means such as a die assembly 128. This die assembly includes upper and lower die shoes 130 and 132 mounted on a press (not shown) which moves them reciprocally toward and away from each other with great force. Various shaped die elements 134 are mounted on the mutually facing surfaces of the die shoes; and when the die shoes are forced together by the press with a sheet metal workpiece between them, the die elements 134 draw the metal workpiece to a given finished configuration.

In order to achieve complex working of the metal workpiece in a single pressing operation, certain of the die elements are movable on the die shoes and are urged toward protracted or extended positions on the shoes by actuating pistons 136 of various cross sectional area. These actuating pistons are in fluid communication via passageways 138 in the die shoes 130 and 132 to the hydraulic line 42 and the pressure unit 10. As the press closes and the die shoes are moved toward each other with greater and greater force, the die actuating pistons and their associated die elements are forced back into their respective die shoes in predetermined sequence according to the cross sectional area of the pistons.

The pressure unit operates to absorb the fluid displaced by the retracting pistons during press closure, while maintaining the high resistance pressures (in the neighborhood of 4500 p.s.i) required for proper metal working; and when the press opens, the pressure unit acts to supply oil to protract the pistons 136 for repositioning the die elements and ejecting the previously finished workpiece.

The system operates in the following manner: When the press opens as in FIG. 2, and the die shoes 130 and 132 are moved apart, there is no retracting force on the actuating pistons 136. Consequently the pressure in the passageways 138, the hydraulic line 42 and in the inletoutlet chamber 38 of the pressure unit 10 falls off toward atmospheric pressure. Similarly, the pressure in the pressure responsive chamber 70 of the control valve spool 62 also becomes decreased, for this chamber is in fluid communication with the inlet-outlet chamber 38 via the internal pressure passageway 92. Consequently, the valve spool bias spring 86, receiving no opposition, moves the valve spool 62 to the left, as in FIG. 2, so that the larger diameter end portion 66 of the spool 62 fully covers the lower connecting passageway 46. When this decreased pressure falls below the pressure in the reservoir 14 however, a force unbalance exists across the check valve element 56 causing it to unseat so that fluid is forced at about 100 to 125 p.s.i. from the reservoir 14, through the upper connecting passageway 44, the inlet-outlet chamber 38 and the hydraulic line 42 to the upper and lower die shoes 130 and 132 where it forces the actuating pistons to their extended position as shown.

When the press is closed and the die elements 134 are brought together against the workpiece, they force back against the actuating pistons 136 causing a pressure increase in the hydraulic line 42 and in the inlet-outlet chamber 38. This increased pressure acts to close the check valve 48. However it is also communicated through the internal pressure passageway 92 to the pressure responsive chamber 70 in the control valve spool 62 and acts upon that end of the spool in opposition to the valve spool bias spring 86. This causes rightward movement of the valve spool which in turn brings about communication between the inlet-outlet chamber 38 and the reservoir chamber 36 via the cutouts 78 in the spool 62. Accordingly, the fluid which is displaced by the retracting pistons during closure of the press is accommodated through the controlled opening of the lower connecting passageway 46 by movement of the valve spool 62.

It is important in many applications, particularly in the hydraulic press situation described herein, to accommodate the rapid volumetric displacements caused by the retracting pistons and yet to maintain the pressure in the system at close to 4500 p.s.i. Also, because various different pistons begin to retract at different points during press closure, the system must be capable of accommodating large surges in both pressure and volume, without the loss of control and valve flutter which often plague conventional valving systems under such conditions.

In the present system, the two larger diameter end portions 64 and 66 of the control valve spool 62 are of equal diameter. Thus, the pressure surges which occur in the inlet-outlet chamber 38 are balanced out at the two end portions of the spool. All control of spool movement occurs as a result of pressure communication through the internal pressure passageway 92 to the end of the spool. Accordingly, the spool does not suffer from the hydrodynamic effects of the fluid rushing through it. Further, the various changes in direction the fluid must take between the inlet-outlet chamber 38 and the pressure responsive chamber 70 causes a dissipation of surge effects so that the valve operates only in response to actual pressure changes and not to surge effects. As stated previously, this surge dissipation is enhanced by the step taper configuration Within the pressure responsive chamber 70.

Because of the semi-cylindrical cut-outs 78 in the valve spool 62, the fluid rushing into the reservoir chamber 36 is diverted in various radial directions to provide a balancing effect. Also, the fluid is conveyed in integral streams as opposed to the spray effect which results where a continuous peripheral opening is used. This serves to reduce tendency toward foaming.

As pressure increases at the left end of the valve spool 62, it is forced rightward against the valve spool bias spring 86. This forces all fluid in the spring seating chamber 72 out through the opening 96 and against the adjustment valve element 100, causing it to open when the pressure reaches a sufficient value, i.e., the value corresponding to 4500 p.s.i. in the inlet-outlet chamber 38. Even while this pressure is sustained, hydraulic fluid passes through the central passageway 74 and the metering orifice insert 76 and into the spring seating chamber 72 where it balances the pressure in the pressure responsive chamber 70. This permits the valve spool bias spring 86 to move the spool 62 back towards the left until the flow through the lower connecting passageway 46 is decreased to a point where the pressure in the system again builds up. The valving arrangement is thus dynamically balanced and in fact is capable of handling pressure and volume variations at rates approaching cycles per second without causing piston bounce, oil foaming or entrapped air.

A modified control valve spool 62' is shown in FIG. 6. This modified spool is similar in overall configuration to the valve spool of FIG. 5 but is somewhat smaller in size, and of course, volumetric capacity. In order to provide a proper relationship between valve movement and passageway opening, the cut-outs 78 of the earlier described spool are replaced by segmentally tapered corners 7 8. These corners may be provided simply by grinding the edge of the right hand larger diameter end por- )n at selected locations to provide the necessary tapered untO'Lll'.

Having thus described my invention with particular ference to the preferred form'thereof, it will be obvious those skilled in the art to which the invention pertains,

'ter understanding my invention, that various changes 1d modifications may be made therein without'departg'from the spirit and scope of my invention, as defined the claims appended thereto.

What is claimed as new and desired to be secured y Letters Patent is:

1. A hydraulic supply system comprising a fluid :servoir, means associated with said reservoir for mainlining the fluid therein at a given pressure, a housing )rmed with a pair of internal chambers, one of said iambers being in open communication with said reservoir nd the other adapted to be connected to a utilization leans, said housing being formed with first and second \ternal passageways, each communicating between said lternal chambers, a check valve arranged in operative ssoci'ation with said first internal passageway and operave to restrict fluid flow through said first passageway a direction from said 'one to the other chamber, a spool alve element having a first land portion movable into nd out of said second passageway from said one chamer, to close and open said second passageway, said spool alve element also having a second land portion extendig out through a similar passageway in the opposite ide of said other chamber, said second land element eing connected to the first land element through a reluced area portion, the cross sectional areas of said ands being substantially equal to neutralize the effects f fluid pressure within said other chamber, means biasng said spool valve element to a position with said first and portion closing said second passageway and means lefining a further fluid passageway communicating beween said other chamber and one end of said spool 'alve element for applying static fluid pressure of said rther chamber to said one end in opposition to said )iasing means.

2. A hydraulic pressure system comprising a fluid 'eservoir, pressure means arranged to maintain fluid in aid reservoir at a first given pessure, means defining an )utput, a housing having a pair of chambers arranged .ide by side therein, one chamber being in communica- ;ion with said reservoir, the other chamber being in :ommunication with said output, means defining first and :econd fluid passageways between said chambers, a check Ialve arranged in said first fluid passageway to open said irst fluid passageway to fluid flow only when the pres- ;ure in said one chamber exceeds that of the other cham- J61, a spool valve element having a pair of axially displaced lands of equal cross section separated by a central portion of reduced cross section, one of said lands fitting :losely in said second fluid passageway, the other land extending into an opening across said other chamber whereby the effects of the fluid in the other chamber upon axial movement of said spool element are balanced, means resiliently biasing said spool element to a position where said one land fully closes said second fluid passageway, and means defining a fluid passageway from said other chamber to one end of said spool valve element to permit static fluid pressure of said other chamber to act in opposition to said bias.

3. A hydraulic supply unit comprising a fluid supply reservoir, a housing having a pair of chambers formed therein, one of said chambers being in direct open and free fluid communication with said reservoir, means defining first and second passageways between said chambers, a check valve in said first passageway and allowing fluid flow from said one chamber into the other chamber, a spool valve having a land portion movable between a first position fully closing said second passageway and a second position partially opening said second passageway, said other chamber being formed with an opening 8 for placing utilization means into fluid communication therewith, spring means biasing said spool valve to said first position, and means defining a fluid passageway communicating betweensaid other chamber and one end of said spool valve, said spool valvebe'ing arranged to be actuated solely by variations} in a static fluid pressure in said other chamber, said chambers being closedto all other means thereby to permit free fluid flow from "said reservoir to said utilization means when the pressure in said utilization means is decreased below that in said reservoir, and to permit rapid return flow from said utilization means to said reservoir while maintaining a controlled static pressure in said utilization means.

4. A valve arrangement comprisinga housing having a pair of chambers formedtherein and separated by an intermediatewall, a spool valve elementarranged for reciprocal movement withinsaid housing .to. openand close a passageway-between said chambers, one endof said valve element extending intoan opening formed in a wall of one of said chambers, said housing being formed with an internal passageway extending from said one chamber to said opening for directingthestatic pressure of said one chamber. to urge said valve element toward a passageway opening position, resilient means acting on said valve element urging it toward a passageway closing positiorfl 'first connection means constructed and arranged to provide free and'op'en flow of fluid between a utilization means and the interior of said one chamber, second connection means constructed and arranged to provide free and open flowof fluid between a reservoir and the interior of the other chamber, saidchambers being closed to all othermeans thereby to permit free fluid flow'from said reservoir to said utilization means when the pressure in said utilization means is decreased below that in said reservoir, and to permit rapid return flow from said utilization means to said reservoir while maintaining a controlled static'pressure' in said utilization means, and check valve means communicating between said chambers for permitting fluid flow from said other chamber to said one chamber. n

5. A valve arrangement as in claim 4 wherein said opening and said passageway are of equal cross sectional area.

6. A valve arrangement comprising a housing having a pair of chambers formed therein and separatedby an intermediate wall having a first opening therethrough, the outer wall of one of said chambers having a second opening therein in alignment. with saidv first opening, a spool valve element having a pair of lands axially separated by an intermediate region of lessercross section, said intermediate region being of an .axial length suflicient to bridge the length of said first opening through said intermediate wall, said lands dimensioned to fit closely within said openingsand being of a length such that at least a portion of one land will remain inside said second opening while the otherland moves axially into and out beyond said first opening, means associated with said housing placing the end of said one land into fluid communication with said one chamber for subjecting the end of said spool valve element closest to said one land to static pressure of said onechamber, resilient biasing means acting on said spool valve element in the opposite axial direction, first connection means constructed and arranged to provide free and open flow of fluid between a utilization means and the interior of said one chamber, second connection means constructed and arranged to provide free and open flow of fluid between a reservoir and the interior of the otherv chamber, said chambers'being closed to all other means thereby to per- 'mit free fluid flow from said reservoir to said utilization means when the pressure in said utilization means is decreased below that in said reservoir, and to permitrapid return flow from said utilization means to said reservoir while maintaining a controlled static pressure in said utilization means, and check'valve means communicating between said chambers for permitting fluid flow from said other chamber to said one chamber.

7. A valve arrangement as in claim 6 wherein said Spool valve element is of generally tubular configuration and formed with grooves in said other land adjacent said intermediate region.

8. A hydraulic supply system comprising a housing formed with a pair of internal chambers separated by an intermediate wall which is formed with a pair of passageways therethrough, one of said chambers being in communication with an outlet, the other of said chambers being in free and open communication with a reservoir, a check valve located in one passageway to open same only when the pressure in said reservoir exceeds that of the outlet, a spool valve element having a pair of axially displaced lands separated by an intermediate region, said lands and intermediate region being of such length as to permit axial movement between a first position with one land closely fitted into said other passageway and the other land closely fitted into an aligned opening across the one chamber, and a second position with said other land remaining at least partially in said aligned opening and said intermediate region bridging said intermediate wall to open said other passageway, said housing further being formed to define a fluid passageway extending from said one chamber to said aligned opening beyond the end of said spool valve element for subjecting said end of said spool valve element to the static pressure in said one chamber, and spring means acting against the other e of said spool valve element.

9. A hydraulic supply system as in claim 8 where said spool valve element is formed with an axially c tending passageway of restricted cross section commu;

5 eating between each end of said spool valve element, pressure relief valve arranged in communication with t end of said axially extending passageway opposite sa aligned opening, and means communicating the out 10 of said pressure relief valve with said other chamber.

References Cited UNITED STATES PATENTS r 726,841 5/1903 Ball 137-1 2,745,429 5/1956 Crookston 251282 2,774,414 12/1956 Machlanski 137494 2,971,524 2/1961 Ruhl 1371i 3,009,480 11/1961 Miller 137625. 9 3,195,571 7/1965 Olsavsky 137-62569 3,234,853 2/1966 Aber 137-209 2,930,398 3/1960 Barrett et al. 1374i WILLIAM F. ODEA, Primary Examiner 25 DAVID J. ZOBKIW, Assistant Examiner US. Cl. X.R. 1 37 4 ss, 493.8, 599, 625.3, 625.37 

